Materials used in the fields mentioned above have some problems. The first problem is the adsorption of proteins, fats and oils, humin, and other substances onto the surfaces of these materials. In the field of separation membranes, for example, there is a problem that the adsorption of such substances causes a decrease in permeation flux. The second problem is insufficient biocompatibility in the medical field, as in artificial organs; the insufficient biocompatibility is causative of thrombus, hemolysis, sensitization, antigen-antibody reaction, etc. The third problem is static build-up, which is serious especially in the field of electronic industry.
A generally employed method for eliminating those problems is to hydrophilize the surface of material. However, this method, in which a surface treatment is conducted to impart hydrophilicity to material surface, is disadvantageous in that the attainable degree of hydrophilicity is low and that the efficiency of material production inevitably decreases due to an increase in the number of steps because of the hydrophilizing treatment which should be conducted after forming.
On the other hand, methods for directly producing a material having hydrophilic surface are to use a polymer having hydrophilic groups as a raw material or coating material, and to incorporate a hydrophilic substance into a material through kneading. For example, methods for the prevention of static build-up are described in Kobunshi Hyomen No Kiso To Oyo (Ge) The Fundamentals and Applications of Polymer Surfaces (the last volume)!, Kagaku Doojin K.K., p.72 (1986), which comprise producing a material having a hydrophilic surface by applying a surfactant to the surface of a material or by incorporating a surfactant into a material through kneading. Besides surfactants, carbon is also used as a hydrophilizing agent incorporated through kneading.
However, such methods based on the incorporation of a hydrophilizing agent through kneading necessarily have the following drawbacks. Since a surfactant should be incorporated in a large amount in order to obtain sufficient hydrophilicity, the surfactant is released from the surfaces of the resultant materials and the materials have considerably impaired properties. On the other hand, the method in which a hydrophilic polymer is used as a raw material or a coating material has problems of dimensional change due to moisture absorption, strength decrease in a wet state, and peeling in wet state from the substrate, although free from the problem of the release of a hydrophilic substance.